A typical cordless telephone system includes a base station and a handset. The base station is typically connected by wire to a central office. The handset communicates with the base station by cordless means, such as through radio frequency signals, without a direct physical connection to the base station. Cordless telephones allow considerable freedom and mobility to the user, allowing the user to move about while engaged in a call, and to conduct a call at a considerable distance from the base station. Presently, two preferred cordless systems exist, digital and analog. Analog systems provide the advantages of simplicity, low cost, and high sound quality, but are vulnerable to in-band interference or jamming. Moreover, analog systems may interfere with other telephones. For this reason, analog systems are restricted in power, resulting in a reduced range.
Digital systems exist, which have both advantages and disadvantages with respect to analog systems. Digital systems provide a digital processing gain, reducing susceptibility to in-band interference, with the resistance to interference increasing with processing gain. Digital systems are less likely to interfere with other telephone systems, and are permitted greater transmit power than are analog systems. The combination of greater transmit power and digital processing gain provides digital systems with a significantly greater range than is possessed by analog systems. Disadvantages of digital systems are a much higher cost due to the need to have powerful processors to perform voice coding and decoding, as well as digital signal processing (DSP) to achieve processing gain. Many parts of a digital system transmitter and receiver, such as a modulator and demodulator, are subject to more stringent requirements than are the corresponding parts of analog systems, and are therefore more complicated. Moreover, digital voice coding, interleaving, and spreading take time to perform, delaying signal transmission. Delay is especially prevalent in systems employing time domain duplexing, in which transmitter and receiver work in turns, half time each. This delay is added to the delay caused by digital processing, and may create a problem called echo. Echo in digital systems produces a significant degradation of sound quality, with the amount and nature of the degradation being influenced by the amount of delay and the spectrum content of the reflection. Echo cancellation in digital systems of the prior art represents a considerable challenge and increases the cost of the system.
An important aspect of transmission in cordless telephone systems is spreading. Systems of the prior art commonly use one of two techniques. These techniques are direct sequence and frequency hopping. In direct sequence systems the information signal is multiplied by a pseudo-random sequence of bits, widening the spectrum of the resulting signal. The received signal is multiplied with the same sequence once again to despread it. Many well-known techniques exist for performing spreading and despreading.
In frequency hopping systems, information bits are grouped into frames. Each frame is transmitted on a different carrier frequency. Over a period of time, therefore, the energy of the signal is spread throughout the entire bandwidth. Frequency hopping systems are appropriate for a time division duplexed system (“TDD”), since the transmitter and receiver of a TDD must be rapidly switched on and off. The transmitter and receiver of a TDD employing a frequency hopping system repeat an on-off cycle, with each repetition of the cycle employing a different frequency. A TDD employing frequency hopping is particularly vulnerable to echo.
It is possible to combine direct sequence and frequency hopping spreading techniques. A system using these combined techniques would have a large processing gain and a high interference immunity, but would be complex and expensive. Due to an extensive processing and hopping delay, it would also be susceptible to echo.
A need therefore exists in the art for a cordless telephone system with a low cost, high processing gain with resulting high interference immunity, and resistance to echo.